The invention relates to a motorized throttle body, particularly a throttle body suitable for being inserted along an intake manifold of an internal combustion engine, to which the discussion which follows will make explicit reference without thereby losing generality.
Motorized throttle bodies are currently known which comprise a feed pipe, through which a comburent fluid (air), or a comburent fluid/fuel mixture reaches the engine's intake manifold; a butterfly valve housed in a movable manner inside the feed pipe and suitable for choking the flow of comburent fluid as a function of its position; and an operating device suitable for selectively moving the butterfly valve to control the flow of the comburent fluid.
FIG. 1 shows the flow curve of a throttle body which uses a complex kinematic mechanism which is required to achieve the equilibrium position at point LH, which defines the state in which the actuator used to move the butterfly valve malfunctions or is not supplied at all because of a breakdown in the current supply unit (limp-home state). In FIG. 1 the angle .alpha..sub.mn, which defines the angle at which there is the minimum flow Q.sub.mn, is close to the minimum angle achievable by the butterfly, whilst the angle .alpha..sub.lh, which denotes the limp-home angle, is positive and greater than the preceding angle. The system proposed to control the angular movement of the butterfly valve is rather complex to operate in that the characteristics of the springs used have a discontinuity around the angle .alpha..sub.lh.
FIG. 2 shows the curve of operation of a negative limp-home system. In this case the minimum flow Q.sub.mn corresponds to zero (.alpha..sub.mn =0) whilst in general the opening of the butterfly valve itay range from -90.degree. and +90.degree.. Although there are undoubted advanstages of simplicity, with this system it is, however, extremely critical to guarantee that the flow at the low point is situated at a very precise value defined by specification; contact between butterfly valve and pipe in correspondence with the angle .alpha.=0.degree. is also possible if the butterfly valve has not been fitted correctly.
From the above it will be clear that even though the system works with the flow curves shown in FIG. 1 or FIG. 2, it is difficult to operate and involves considerable complications in the electronic devices used to control it.